A tire dynamic loaded radius is changed by a load. When the load becomes heavier, the dynamic loaded radius becomes smaller, and accordingly, a wheel speed is increased. Regarding such a load sensitivity, a formally established definition does not exist. However, in general, a change ratio of the dynamic loaded radius to a fixed amount of a load change serves as the load sensitivity.
This load sensitivity is conventionally used for an alarm apparatus of a tire having a decreased air pressure or used for estimating a decreased pressure sensitivity of the dynamic loaded radius (decrease ratio of the dynamic loaded radius to a fixed rate of a decreased pressure) (see Patent Literatures 1 to 3, for example).
The load sensitivity is measured as an inclination of a relational expression of a lateral acceleration (lateral G) and a speed difference between left and right wheels of a vehicle. Thus, the load sensitivity can be measured only when the vehicle makes a turning running. The load sensitivity is not a fixed value but has a speed dependency changed by a vehicle speed. Therefore, there is a need for acquiring the load sensitivity in accordance with the vehicle speed. Thus, in order to measure the load sensitivity at a certain speed, there is a proposed measure for, for example, providing a speed window. However, even in this case, there is a need for the turning running of the vehicle at the speed.